There are numerous methods of providing vertical lift for an aircraft. Of particular interest here are the supply of gaseous fluid ducted from a gas turbine propulsion engine exhaust gas stream or by-pass air stream or from both to one or more vectorable nozzles positioned at appropriate places in the aircraft. The exhaust from these nozzles can be vectored to provide vertical lift and forward thrust. In another method one or more dedicated lift engines are installed in the aircraft with their axes substantially vertical to the aircraft longitudinal axis in order to provide lift thrust only. A separate propulsion means is used for forward thrust in conventional wing-borne flight.
There has also been proposed a further method which combines these principles using one or more dedicated lift fans driven from the main propulsion engine by means of an exhaust gas offtake or a mechanical shaft drive. The Ryan Aeronautical Company built and flew a fan-in-wing aircraft in the early 1960's in which power supply to the fans was by gas drive derived from the main propulsion engines. A diverter valve in the exhaust ducts of the two propulsion engines could be switched as required to divert the entire efflux from both engines to supply lift power. The wing fans exhausted through exit louvres which could be angled to convert some of the lift thrust to forward thrust. In addition the louvres could be turned flat to lie flush with the wing undersurface when the lift fan was not in use. The configuration included a smaller nose fan, also gas powered, which exhausted through a louvred aperture in the underside of the aircraft nose which, when not in use, was closed by a pair of longitudinally hung bay doors.
International Patent Application No WO 91/17083 published Nov. 14, 1991 in the name of Lockheed Corporation also described a lift fan arrangement. In this proposal a lift fan is driven by a shaft which may be selectively clutched directly to the low pressure spool of a main propulsion gas turbine engine to supply lift fan power. The lift fan exhausts through a louvred aperture in the underside of the fuselage. However, these doors have two positions only and are either open or closed. Since the main engine exhaust is diverted in a shaft driven arrangement thrust vectoring for transition to forward flight is provided by a telescoping elbow type nozzle at the end of the main engine jet pipe.
These and other arrangements which provide vertical lift selectively during the take-off and landing flight phases must do so without excessively heavy ancilliary equipment, and keep the powerplant and installation as light and compact as possible. The composite powerplant which supplies power for both lift and wing-borne flight appears a good solution providing acceptable transition performance between vertical and forward flight is achieved.